The processes of formation of carbide-free bainite and the effect of the temperature-time parameters of isothermal hardening in the range of bainitic transformation on the structure and impact toughness of low-alloy structural steels are studied. A method for nondestructive testing of the impact toughness of medium-carbon steels heat-treated by isothermal quenching is developed on the basis of identified patterns of changes in the properties of retained austenite and in the impact toughens of the steels.